The present invention relates to a shipping and/or retail package for and with food(s) that are intended for consumption while hot and are to be heated in the package according to the definition of the species of patent claim 1.
The present invention also relates to a method of preparing foods in a container in a microwave oven, where a steam buildup situation develops with an excess pressure in the container during heating due to the water content of the foods and the construction of the container.
A package and a cooking method of this type are known from International Patent No. WO 99/32373. In particular, fresh foods which are still raw can be cooked in just a few minutes in the known package by using this method, thus making it possible to achieve a quality that is impossible with traditional prepared dishes.
The valve plays an important role here. The valve is designed as one-way non-return valve and permits automatic venting of the foods before the actual preparation with an extremely low opening pressure of just a few millibar. At the same time, it ensures a hermetic seal by closing up again after each opening as soon as the internal excess pressure has dropped below a certain level.
International Patent No. WO 99/32373 describes a valve available under the brand name of WICOVALVE from Wipf AG at CH-8604 Volketswil. This valve was designed for prefilled bags of roasted coffee. The valve has a dimensionally stable valve body with a flat sealing face and a thin, flexible membrane in it. The membrane is wetted with some sealing oil. Because of this design, the valve seals well and it opens and closes within a relatively narrow tolerance. Thus, its opening pressure is only approx. 3 mbar. It closes again when the internal excess pressure has dropped to approx. 0.5 mbar. The valve also has a well-defined flow cross section. Because of these properties, the cooking results can be monitored and predicted with sufficient accuracy. Thus, for example, vegetables packaged in this type of package can be cooked precisely until they have reached an xe2x80x9cal dentexe2x80x9d condition.
The known package is preferably designed as a disposable package. In one embodiment, it consists of a flat shell made of a plastic material that is relatively thin but still has enough dimensional stability to withstand shipping and handling as well as the temperature and pressure stresses occurring during the steam buildup phase. The shell is covered with a transparent, flexible plastic film which is welded to the shell along its peripheral edge. The valve is preferably also incorporated into this cover film.
The purpose of the present invention is to further improve the known package and the method associated with it. It has been found that a reduced pressure situation can under certain conditions occur in the package during cooling to the desired temperature for eating after being heated in a microwave oven in particular. In the case of packages with wall parts that are dimensionally stable although still relatively thin, such as the above-mentioned shell, this may lead to an unwanted deformation xe2x80x9cinto the negative.xe2x80x9d Due to the design of the valve according to the present invention so that it undergoes changes during heating of the foods in the package so that the valve can no longer close again, at least while the food is cooling to the temperature for consumption, the development of a reduced pressure in this phase is prevented. Of course, the valve may also undergo permanent changes, because its function is no longer needed after preparation of the food.
With a valve of the above-mentioned type, the desired change can occur in particular by selecting for the thin membrane a plastic material which undergoes deformation at the resulting temperature when the package is heated. However, this deformation should occur only at a temperature of at least approximately 50xc2x0 C. or even 60xc2x0 C., so the valve retains its function before the actual heating process, i.e., during shipping and storage of the package.
In order for the desired steam atmosphere to be able to develop in the package during heating, the foods should have a water content of at least 30%, preferably 40%. The foods are preferably preseasoned to their final taste, so that they are ready to eat immediately after heating in a microwave. The term xe2x80x9cseasoningxe2x80x9d should be understood to refer to the usual herbs, spices and flavorings as well as the addition of a small amount of herb butter, sauce or the like. Servings of meat or fish are preferably also coated with an emulsion of seasonings, an oil- or fat-based marinade or the like, additionally protecting the servings of meat or fish from becoming stringy in the microwave. Raw dough for a cake or the like may also be used, in which case it is then xe2x80x9csteam-bakedxe2x80x9d so to speak in the steam atmosphere.
In another aspect, the present invention provides a method for heating a food product by microwave energy that includes the steps of providing a food product having a water content that enables steam pressure to develop during heating by microwave energy; disposing and sealing a food product in a microwavable shipping and/or retail package, the package comprising a container with a one-way valve mounted in a wall of the container, the valve having a flexible membrane; and heating the package by microwave energy, causing excess pressure to develop in the container, whereby the valve automatically opens and vents pressure to the outside and the valve automatically closes when the pressure drops, wherein the valve may automatically open and automatically close until the membrane deforms, thereby causing the valve to remain open.
Advantageous embodiments and refinements of this invention are characterized in the dependent claims.